AGGREGATED TRANSMISSION IN WLAN SYSTEMS WITH FEC MPDUs

ABSTRACT

In various embodiments, a wireless device may determine the quality of a channel by transmitting at least one packet to another device and receiving from that other device an indicator of the quality of the channel. Based on the quality indicator, the device may determine an estimated packet error rate, and subsequently transmit few enough packets that if the estimated percentage of those packets fail, there will be time to retransmit them.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 11/529,986,filed Sep. 29, 2006, and claims priority to that date for all applicablesubject matter.

BACKGROUND

Recent developments in a number of different digital technologies havegreatly increased the need to transfer large amounts of data from onedevice to another or across a network to another system. Technologicaldevelopments permit digitization and compression of large amounts ofvoice, video, imaging, and data information, which may be rapidlytransmitted from computers and other digital equipment to other deviceswithin the network. Computers have faster central processing units andsubstantially increased memory capabilities, which have increased thedemand for devices that can more quickly transfer larger amounts ofdata.

These developments in digital technology have stimulated a need forutilizing the spectrum used for wireless interconnection within thesenetworks and controlling the protocol overheads prior to actualtransmission of a message. Various overheads have been proposed toprovide improved performance in transmission times in networks thatadhere to the emerging protocol standards. Further improvements intransmissions, such as in data packet transmissions, are needed.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 is a diagram that illustrates a wireless device that incorporatescircuitry and an algorithm to efficiently transmit aggregated datapackets in accordance with the present invention;

FIG. 2 is a timing diagram that illustrates aggregated data packets inaccordance with the present invention; and

FIG. 3 is a flow diagram that illustrates a method of determining anumber of FEC MPDUs to include with MPDUs to improve transmission ofaggregated data packets in accordance with the present invention.

It will be appreciated that for simplicity and clarity of illustration,elements illustrated in the figures have not necessarily been drawn toscale. For example, the dimensions of some of the elements may beexaggerated relative to other elements for clarity. Further, whereconsidered appropriate, reference numerals have been repeated among thefigures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention.

As shown in FIG. 1, wireless communications device 10 includes a radioto allow communication in an RF/location space with other devices.Accordingly, communications device 10 may operate in a wireless networksuch as, for example, a Wireless Local Area Network (WLAN), a WirelessPersonal Area Network (WPAN), or a combination thereof. Communicationsdevice 10 is any type of wireless device capable of communicating in anRF/location space with another device that is capable of using analgorithm that monitors and controls packet aggregation techniques toimprove the performance and effective throughput in networks.

The figure illustrates a transceiver 12 that both receives and transmitsa modulated signal from multiple antennas. The illustrated embodimentfor a Multiple-in, Multiple-out (MIMO) system utilizes multiple antennasat both the transmitter and receiver side to provide that independentdata streams are simultaneously transmitted from different antennas. Theuse of this MIMO system may take advantage of spatial multiplexing toincrease wireless bandwidth and range in providing a significantcapacity gain over conventional single antenna systems. The MIMO systemmay comprise a number of types of antenna including an omni-directionalantenna, a directional antenna, or high-gain antennas, among others, andeven a combination of antenna types. The omni-directional antennas maybe used for line-of-sight communications with mobile stations spread inall directions. The directional antennas may transmit and receive RFenergy more in one direction, and the higher-gain antennas may provide anarrower radiation beam width. However, it should be noted that neitherthe type of antennas nor the arrangement of antennas used to implementthe MIMO algorithm should be considered a limitation of the presentinvention.

A spectrum management and channel characterization block 14 illustratedin FIG. 1 may be used to monitor characteristics of selectedcommunications channels and adjacent channels. Block 14 may cognitivelymonitor channel characteristics such as channel signal power and othercharacteristics to gather parameters and make determinations about thequality of the selected channel and the adjacent channels. Block 14 mayuse the channel parameters to switch transmission energy profiles inaccordance with deteriorating channel conditions. Further, block 14 maycalculate an indicator value of the channel quality directly from areceived packet. Alternatively, at least one packet may be transmittedover a channel and an indicator value of the channel quality based onthe packet transmitted may be received in return. A modulation andcoding scheme is then selected for data packets transmitted over thechannel that would achieve a desired statistical packet error rate basedon the channel quality indicator value. Data packets may be collectedusing the selected modulation and coding scheme to create an aggregateddata transmission.

In some embodiments the transmitted data may be left unprotected fromrandom channel impairment and then it is desirable to include a linkadaptation 16 to improve the system performance and quality of service.Link adaptation in wireless communications denotes the matching of themodulation, coding and other signal and protocol parameters to theconditions on the radio link. The dynamic link adaptation processupdates signal and protocol parameter changes as the radio linkconditions change (e.g. the path loss, the interference due signalscoming from other transmitters, the sensitivity of the receiver, theavailable transmitter power margin, etc.).

Analog front end transceiver 12 may be a stand-alone Radio Frequency(RF) discrete or integrated analog circuit. Transceiver 12 may also beembedded with a processor as a mixed-mode integrated circuit. Theprocessor, in general, processes functions that fetch instructions,generate decodes, find operands, and perform appropriate actions, thenstores results. The processor may include baseband and applicationsprocessing functions and utilize one or more processor cores 20 and 22dedicated to handle application specific functions and allow processingworkloads to be shared across the cores. The processor may transfer datathrough an interface 26 to a system memory 28 that may include acombination of memories such as a Random Access Memory (RAM), a ReadOnly Memory (ROM) and a nonvolatile memory, although neither the typenor variety of memories included in system memory 28 is a limitation ofthe present invention.

Wireless communications device 10 may operate in a network where aCSMA/CA protocol is used. The Carrier-Sense, MultipleAccess/Collision-Avoidance (CSMA/CA) protocol is a network-contentionprotocol that listens to the network to avoid transmission collisions.Before data signal transmissions and packet delivery across the network,the CSMA/CA protocol broadcasts a signal onto the network to listen forcollisions in order to indicate to other devices to refrain frombroadcasting.

The architecture of wireless communications device 10 includes, amongother layers, a Media Access Control (MAC) layer and a PHY layer. TheMAC level operates a MAC aggregated management block 18 to control MACprotocol data units (MPDUs) that dictate the process for moving datapackets to and from an interface across a shared channel while the PHYlayer provides the hardware for sending and receiving bit stream signalson a carrier through the network. Wireless communications device 10employs functional logic and various methods in the MAC layer todynamically create and send information MPDUs in an aggregatedtransmission along with packet level FEC MPDUs. Forward Error Correction(FEC) is a system of error control for data transmission, a techniquethat allows the receiver to correct errors in the currently receiveddata.

Based on channel characteristics gathered in spectrum management andchannel characterization block 14, the MAC layer generates the MPDUsbeing aggregated for transmission and a sufficient number of FEC MPDUs.After transmitting the aggregate data and receiving acknowledgements forcorrectly received packets, the data packets that were not correctlyreceived are retransmitted in the remaining available time. Thus, thenumber of packets to transmit in the aggregated data transmission isbased on the expected packet error rate such that there is timeavailable after the transmission to retransmit the expected failedpackets. Further, the number of FEC MPDUs transmitted with the aggregatedata may be based on the expected probability of packet errors in thechannel.

FIG. 2 illustrates one aggregate data packet denoted as an aggregatedMPDU 200 (A-MPDU, Aggregated-MAC Protocol Data Unit). Individual MPDUs210, 220 and 230 are each prefixed with an aggregate framing header andconcatenated in one aggregated protocol data unit denoted as A-MPDU 200as shown. MPDUs 210, 220 and 230 may be aggregated at the MAC inhardware or in software. One or more of the aggregated MPDUs may be FECMPDUs 230.

In general, the MAC header contains details of the MPDU that principallyinclude the transmit and receive address that identify the source anddestination of the packet on the wireless link, miscellaneous controlinformation and payload encryption information. The payload may containeither a management message or user data. A payload in a transportconnection may contain a MAC service data unit (MSDU), fragments ofMSDUs, aggregates of MSDUs, aggregates of fragments of MSDUs, bandwidthrequests or retransmission requests according to the MAC rules onbandwidth requesting, fragmentation, and packing.

By aggregating data packets, a single block acknowledgement may to besent for several messages to eliminate the overhead of interframespacings or physical layer preambles and acknowledgements that accompanyeach individual packet. However, an aggressive implementation ofaggregation may impact overall performance, specifically inimplementations which use the same physical layer preamble field whichhas been used to estimate the channel for the entire duration of theaggregated packet. Put another way, a block acknowledgement sent as aresponse regarding the proper delivery of the packets that is delayedfor a long duration causes a slow link adaptation compared totraditional 802.11a/b/g MACs where an acknowledgement is expect for eachMPDU that is transmitted as a PHY packet.

In addition, retransmissions of MPDUs that were not receivedsuccessfully may be scheduled. The retransmissions have delays due tothe aggregated delivery of MPDUs because any retransmissions of an MPDUwait until all MPDUs in the aggregated transmission are delivered, theblock acknowledgement is received, and the next opportunity fortransmission becomes available. Immediate retransmissions may beattempted in the same TxOP (Transmit Opportunity) time but it could bedemanding on the hardware. It is also likely that for large aggregatedpacket sizes, trailing packets within the aggregated packet may haveerrors if the channel estimation becomes invalid as time progressesduring data transmission of the aggregated packet. Thus, it is desirableto have an aggregated packet length and transmission duration such thatthe estimation of the channel continues to remain valid and packets arenot determined to be in error due to an incorrect estimation of thechannel being used for decoding at the receiver.

FIG. 3 shows a flowchart in accordance with various embodiments of thepresent invention that illustrate an algorithm in accordance with thepresent invention that may be used to improve the performance andeffective throughput in WLAN networks that employ packet aggregationtechniques. Method 300 or portions thereof are performed by theprocessor/transceiver combination of an electronic system. Method 300 isnot limited by the particular type of apparatus, software element, orsystem performing the method. Also, the various actions in method 300may be performed in the order presented, or may be performed in adifferent order. Further, in some embodiments, some actions listed inFIG. 3 may be omitted from method 300.

Method 300 is shown beginning at block 310. In block 320 the receiverportion of a device such as, for example, an 802.11 device or an 802.11derivative device receives a transmission. The received transmissionincludes an aggregate indication as indicated in the header of theA-MPDU 200. In block 330 a determination is made as to whether one ormore received frames have Ack_Policy equal to BlockAck. Ack_Policydenotes which type of acknowledgement is to be used such as, forexample, a block acknowledgement, a regular acknowledgement or noacknowledgement. In block 340 a determination is made as to whetherBlock_Ack_Policy is equal to FECMode. This may be explicitly indicatedor inferred by the presence of FEC MPDUs in the aggregate.

Method 300 further shows in block 370 that if the number of receivedframes in error is less than or equal to the number of correctFECFrames, a BAack response to indicate a successful transmission forall frames is sent. The response includes the number of frames that arein error for feedback information to the transmitter. In block 380 theresponse includes a BAck signal that indicates the latest received errorframes that cannot be corrected with correctly received FEC frames inthe response. Again the response may also indicate the number of framesin error for feedback information to the transmitter.

The various embodiments for this invention involve combinations of linkadaptation, aggregation, retransmissions and FEC in order to provideoptimized transmissions at the physical layer. In some embodiments linkadaptation may be done prior to aggregated transmission. Link qualitymay be estimated based on the success or failure of previoustransmissions as indicated by the feedback from the receiver. It isunderstood that wireless communications device 10 provides wirelesstransmissions to the channel knowing that some packets in the aggregatedtransmissions may be received by the receiving device with packeterrors. Knowing that errors may occur, wireless communications device 10may include redundant FEC MPDUs along with data MPDUs to match theexpected error rate of transmissions of packets. Thus, wirelesscommunications device 10 dynamically adjusts the combined number ofredundant FEC MPDUs and data MPDUs to match or exceed the expected errorrate of transmissions of packets in order to alleviate the need forretransmissions.

Based on the modulation and coding schemes chosen for transmission andthe available transmit opportunity time TxOP, wireless communicationsdevice 10 may estimate the number of bits that should be sent in theTxOP time period. Wireless communications device 10 may use the transmitopportunity time TxOP to provide flexibility in providing longer accessto the medium while transmitting higher priority traffic across thechannel and shorter access to lower priority traffic. The modulation andcoding schemes chosen by wireless communications device 10 may be usedto decide how many packets may be aggregated based on the packet size. Ablock acknowledgement request is sent after the transmission of theaggregated packets and a block acknowledgement is received in response.

As mentioned, based on the expected physical layer packet error rate,wireless communications device 10 may include FEC MPDUs along with theMPDUs for transmission in the aggregated packet transmission. By way ofexample, if k data MPDUs and (n−k) FEC MPDUs are transmitted in anaggregated packet (n and k are integer values), then as long as any k/nMPDUs are received the aggregated transmission is considered successful.Again, by including redundant FEC MPDUs based on Reed-Solomon coding inthe aggregated packet, retransmissions may be avoided. The BlockAcknowledgement (BlockAck) signal received in response to thetransmitted aggregated packet provides information about which MPDU's inthe A-MPDUs are in error.

Method 300 may be extended to include further steps as illustrated bythe following example. The transmitting node may send 10 MPDUs and 2 FECMPDUs for a total of 12 MPDUs. Upon receiving the aggregatedtransmission the receiving node may determine that three MPDUs are inerror. In the receiving node, the two FEC MPDUs would cover for two ofthe MPDUs received with errors. The receiving node may send a responsein its BlockACK signal to the transmitting node indicating the errors inthe three MPDUs. The transmitting node may create and make available anumber of additional FEC packets/MPDUs that are kept ready during thetransmission of the 12 MPDUs. When the transmitting node receives theBlockACK that indicates more MPDUs were in error than the number of FECMPDUs originally sent, the transmitting node may send one additional FECMPDUs if time is available in the transmit opportunity time TxOP.

This immediate ability by the transmitting node to respond to the errormessage from the receiving node is applicable if the MAC protocol allowsfor an immediate retransmission when the BlockACK signal arrives,otherwise it would not be applicable. In this example the transmittingnode does not have to fetch a specific packet/MPDU that is in error.Rather, the transmitting node would send an FEC packet/MPDU that itcreated and kept available for transmission in the immediateretransmission phase in the MAC protocol.

Note that if the channel coherence time and channel estimate do notremain valid for the duration of transmission and a burst of trailingMPDUs are detected to be in error, wireless communications device 10 mayscale back on the number of packets being aggregated. On the other hand,if the trailing MPDUs are determined to not be in error duringtransmission based on the received block acknowledgement signal, thenadditional MPDUs may be aggregated into the A-MPDU. Further, wirelesscommunications device 10 may receive link adaptation feedback from areceiving device with suggestions of modulation and coding choices andcorresponding PHY packet error rates.

By now it should be apparent that cognitive radio networks mayincorporate features of the present invention to improve over-the-airtransmissions. A wireless communications device may receive an indicatorvalue indicative of the quality of a communications channel. Thereceived indicator value is used to calculate an expected errortransmission rate in a Media Access Control (MAC) layer of the wirelesscommunications device. The wireless communications device selects amodulation and coding scheme to transmit data packets over the channelbased on the expected error transmission rate. The wirelesscommunications device dynamically adjusts the number of redundant FECMPDUs to transmit in the aggregated data packets based on the expectederror transmission rate and the chosen modulation and coding choices.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those skilled in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

1. A method of communicating, comprising: transmitting at least onepacket over a channel and receiving in return an indicator value ofchannel quality based on the at least one packet; selecting a modulationand coding scheme for subsequent packets transmitted over the channelthat would provide a predetermined statistical packet error rate basedon the indicator value of channel quality; using the selected modulationand coding scheme to create a data transmission; and determining anumber of subsequent packets to transmit in the data transmission basedon the expected packet error rate such that there is time availableafter the transmission to retransmit expected failed packets.
 2. Themethod of claim 1, further comprising transmitting the subsequentpackets.
 3. The method of claim 1, further comprising receiving anacknowledgement to the subsequent packets.
 4. The method of claim 2,further comprising retransmitting the subsequent packets that actuallyfail.
 5. An apparatus, comprising a wireless device having a transceiverand a processing device, the wireless device to: transmit at least onepacket over a channel and receive in return an indicator value ofchannel quality based on the at least one packet; select a modulationand coding scheme for subsequent packets transmitted over the channelthat would provide a predetermined statistical packet error rate basedon the indicator value of channel quality; use the selected modulationand coding scheme to create a data transmission; and determine a numberof subsequent packets to transmit in the data transmission based on theexpected packet error rate such that there is time available after thetransmission to retransmit expected failed packets.
 6. The apparatus ofclaim 5, wherein the wireless device is further to transmit thesubsequent packets.
 7. The apparatus of claim 5, wherein the wirelessdevice is further to receive an acknowledgement to the subsequentpackets.
 8. The apparatus of claim 6, wherein the apparatus is furtherto retransmit the subsequent packets that actually fail.